The Mobilization and Transport of Particles and Particle-Associated Contaminants in the Unsaturated Zone

进入自然环境的许多有害污染物与土壤表面结合很强；因此，它们的迁移性受到吸附的限制。土壤中粘土和粉粒大小的颗粒主要负责污染物的吸附，因为它们的表面积很大。在非饱和区，这些颗粒容易被渗透的降雨所动员和运移。如果这些颗粒被动员起来，相关污染物的迁移将会增加。要使与颗粒有关的污染物的传输变得重要，必须满足三个先决条件：(1)必须有颗粒源(动员)，(2)污染物必须吸附到颗粒上(吸附)，以及(3)颗粒必须向下迁移。这项研究计划着眼于提高对非饱和区颗粒物来源或迁移的理解。我们假设，控制非饱和区颗粒物迁移的两个主要因素：(1)土壤结构的异质性和(2)降雨事件的特征。土壤结构的非均质性导致优先的流动路径，或通过低渗透性基质的高渗透性路径。在土壤中，这些优先流动的路径通常被称为大孔隙。大孔隙通常是由土壤干燥、根生长和洞穴动物形成的。渗入的水以比周围土壤基质中的速度大得多的速度通过大孔隙。速度越大，附着的颗粒的剪切力越大，颗粒的动员程度越大。对于降雨，强度、持续时间和频率很重要。降雨的强度将控制水流通过土壤的速度，从而控制颗粒动员的程度。降雨事件的持续时间将影响颗粒动员的动力学，导致从入渗水的初始脉冲期间的快速颗粒释放转变为较慢的稳态速率。降雨事件的频率将控制颗粒供应的再生。研究计划概述了在模型系统和真实土壤中对这些假设的测试。模型系统将由已知大小和几何形状的颗粒和介质组成。真正的土壤将从颗粒促进污染物运输可能显著的地点提取。测量的重点是被动员的颗粒的数量、大小和组成。对这些关键颗粒特性的了解将有助于对污染物吸附和颗粒传输的评估。

9909553RyanMany of the hazardous contaminants introduced into the natural environment bind strongly to soil surfaces; hence, their mobility is limited by sorption. Clay-and silt-sized particles in soils are primarily responsible for the sorption of sorption of contaminants owing to their high surface areas. In the unsaturated zone, these particles are susceptible to mobilization and transport by infiltrating rainfall. If these particles are mobilized, the migration of the associated contaminants will increase. For transport of contaminants associated with particles to be significant, three prerequisites must be met: (1) there must be a source of particles (mobilization), (2) contaminants must sorb to the remain sorbed to the particles (sorption), and (3) the particles must migrate downward. This research plan focuses on improved understanding of the source, or mobilization, of particles in the unsaturated zone.We hypothesize that two major factors control the mobilization particles in the unsaturated zone: (1) heterogeneity in soil structure and (2) characteristics of rainfall events. Soil structure heterogeneity leads to preferential flow paths, or routes of high permeability through a matrix of lower permeability. In soil, these preferential flow paths are often referred to as macropores. Macropores are typically formed by soil desiccation, root growth, and burrowing fauna. The flow of infiltrating water is funneled through macropores at velocities much greater than those occurring in the surrounding soil matrix. The greater velocities lead to greater shear on attached particles and greater particle mobilization. For rainfall, the intensity, duration, and frequency are important. The intensity of rainfall will control the velocity of water flow through the soil and, hence, the extent of particle mobilization. The duration of rainfall events will affect the kinetics of particle mobilization, causing a change from a rapid particle release during the initial pulse of infiltrating water to a slower steady-state rate. The frequency of rainfall events will control the regeneration of the particle supply.The research plan outlines tests of these hypotheses in model systems and real soils. The model systems will consist of particles and media of known size and geometry. The real soil will extracted from sites at which particle-facilitated transport of contaminants may be significant. Measurements focus on the amount, size, and composition of the mobilized particles. Knowledge of these key particle properties will aid in assessment of contaminant sorption and particle transport.